There is emerging evidence that distinct populations of actively cycling and quiescent intestinal stem cells (ISCs) co-exist within the small intestine epithelium and function cooperatively in tissue renewal. This proposal explores the hypothesis that these ISC populations are regulated by distinct sets of extracellular signals involved in organ size control and oxygen-sensing. This hypothesis is evaluated by three Specific Aims: (1) to explore the Hippo signaling pathway, implicated in controlling organ size, in regulating the regenerative responses of active and quiescent ISC populations, (2) to examine metabolic regulation of active and quiescent ISCs by the oxygen-sensing pathway, and (3) to determine the molecular signatures of active versus quiescent ISCs by comparative gene expression profiling. These Specific Aims will be achieved by employing rigorous mouse genetics to examine ISCs in vivo within their stem cell niches and validated ex vivo clonogenic culture techniques that allow for sustained proliferation and multi-lineage differentiation of single isolated ISCs. Achievement of these Aims should provide insight into the molecular differences between active and quiescent ISCs at the transcriptional level and their differential regulation by diverse pathways governing organ size and metabolism.